Many modern wireless systems operate in reuse-1, that is, all cellular cells in the deployment use the same operation frequency. For this purpose a cell is typically defined as a radio entity, corresponding to transmission of one sector over one carrier. A site is defined as a geographical entity (i.e. location), which typically contains 1-6 sectors, and thus has at least 1-6 cells (the number of cells exceeds the number of sectors if more than one carrier is available). Using the same frequency reduces the frequency resources required to deploy a cellular service, however it also causes interference between adjacent cells. To overcome the interference, modern wireless international standards suggest utilizing different tools, one of the primary tools is to assign to each cell a different spreading sequence (CDMA) or permutation (OFDMA). One of the problems associated with the use of an International Recommendation such as the WiMAX Forum Mobile System Profile, is, that while it allows efficient reuse-1 operation for transferring data by using different permutations, the WiMAX maps (control channel) are not similarly equipped to handle reuse-1. The control channel in WiMAX defines areas in a two dimensional region of frequency subchannels and time domain symbols. A subchannel is a logical definition of a granular unit in the frequency domain.
In order to comply with various standards such as the WiMAX, the compatible signals are divided into fixed time frames, where each frame comprises several parts. The frame MAP zone—a control channel describing the following Down Link and Uplink data bursts in addition to other signals constituting the frame; Down Link Zone—containing Data bursts from base station (BS) to mobile station (MS); Feed Back zone—containing various mobile station (MS) to Base station (BS) signaling; and finally Uplink Zone—containing data bursts from MS to BS. For all zones except for the MAP zones, different cells may use different permutations.
Permutations are one of the tools used by OFDMA to provide averaging of interference. Each BS may use all OFDMA tones for transmission, but orders them differently. Now let us assume that the first BS OFDMA transmission is divided into many sub-channels (groups of tones), each of them transmitted to a different MS (associated with that first BS) with different power level (e.g. according to path loss and interference encountered by the MS). An interfered MS belonging to a second BS receives a sub-channel from second BS but is interfered by the signal sent by the first BS. By permuting differently the tones and first BS and second BS, the tones interfering the sub-channel received by interfered MS will belong to many different sub-channels of the first BS. Therefore it is unlikely that the interfered sub-channel will be hit only by high power interference, and typically it would be hit by many different interference power levels (from different sub-channels), and therefore interference would be averaged. This desirable averaging effect occurs due to different MS transmitting with different permutations. The interference averaging effect could have been useful for frame MAP (control channel) data as well since the frame MAP is typically transmitted on a subset of all available tones. The other tones (not transmitting the frame MAP) may be either quiet or be used to transmit lower power data bursts. Therefore by interference averaging achieved through different permutations, the interfered MS receiving its frame MAP, will be partially hit by lower power non-map transmission of other interfering BS which will be easier to handle.
However, since WiMAX frame Maps use the same permutation for all base stations, this desirable interference averaging effect does not occur. Other tools available to counter interference are scheduling the MS transmission only at times when its CINR (Carrier to interference and noise ratio) is high, and/or using retransmissions (HARQ, hybrid ARQ). Unfortunately, none of these techniques are applicable to frame MAP transmission. Frame MAPs must be transmitted all the time to all of the MSs, therefore scheduling is not the answer. Moreover while the frame MAPs provide the retransmission mechanisms (HARQ) for protecting the data bursts, they are not themselves protected by a retransmission mechanism. In other words frame MAPs should be reliably received on the first reception.
A mechanism is therefore needed to allow WiMAX to be deployed in reuse-1 for frame MAPs as well as for data.